Recently, the downsizing and performance improvement of the electronic devices are under rapid development. Further, the improvement of the transmission speed and the lowering of driving voltage in a circuit component in use progress remarkably as seen in the high-speed transmission systems such as the antenna circuit in the cell phone, the RF module, USB 2.0, and USB 3.0, S-ATA2, HDMI (registered trademark) and the like. Unfortunately, with the downsizing of the electronic device or the lowering of driving voltage of the circuit component, the breakdown voltage of the electronic components which are used in the electronic devices decreases. In this respect, to protect the electronic components from overvoltage becomes an important technical subject, for example, to protect the electronic components against the electrostatic pulses generated when the human body is brought into contact with the terminals of an electronic device.
In the past, in order to protect the electronic components from such electrostatic pulses, a method of providing a ceramic varistor between the ground and a line into which the static electricity enters has generally been used. However, the ceramic varistor usually has a relatively high electrostatic capacity which is the main cause for the deteriorated signal quality when the ceramic varistor is used in the high-speed transmission system. In addition, an ESD protection device having a large electrostatic capacity cannot be used in an antenna circuit or an RF module. In this respect, an ESD protection device with a small electrostatic capacity is required to be developed.
An ESD protection device has been suggested as the ESD protection device having a low electrostatic capacity which is provided with two separately and oppositely disposed electrodes, wherein a composite of a conductive inorganic material and an insulating inorganic material is arranged between the electrodes as a discharge inducing section. Similar to the ceramic varistor, such an ESD protection device is also disposed between the ground and a line into which the static electricity enters. If a much too high voltage is applied, discharge will occur between the electrodes of the ESD protection device and then the static electricity will be introduced into the ground side.
Such an ESD protection device provided with the gap typed electrodes possesses characteristics such as a high insulation resistance, a small electrostatic capacity and a good responsiveness. In another respect, a problem rises that the electrodes and the conductive inorganic material inside the discharge inducing section will agglomerate due to the heat or stress generated during the discharge process to cause a short circuit.
As a technique to inhibit the short circuit derived from discharge process, for instance, Patent Document 1 has disclosed an ESD protection device, wherein a mixture portion is disposed between an opposing portion of the oppositely disposed electrodes and the multilayered ceramic substrate. Such an ESD protection device is disclosed, which have the mixture portion containing a metallic material having a shrinkage behavior during firing that is the same or similar to that of the material for the opposing portion of the oppositely disposed electrodes and also a ceramic material having a shrinkage behavior during firing that is the same or similar to that of the material for the multilayered ceramic substrate.